Histopathology of Cutaneous Vasculitis

Vasculitis is an inflammatory process occurring primarily on the vessel wall. It results in vessel wall destruction and subsequent hemorrhagic and ischemic events. Skin is the most common target organ for vasculitis.1,2,3,4 Cutaneous vasculitis includes vasculitis of the dermal small vessels5 and subcutaneous small muscular vessels.6 It can be a self-limited benign disease restricted to the skin or be a significant component of many systemic vasculitic syndromes such as collagen diseaseassociated vasculitis or anti-neutrophil cytoplasmic antibody (ANCA)-associated primary vasculitic syndromes. Cutaneous vasculitis most frequently manifests as palpable purpura or infiltrated erythema (indicating dermal small vessel vasculitis), and less frequently as nodular erythema, livedo racemosa, deep ulcers, or digital gangrene (indicating deep dermal vasculitis or subcutaneous muscular-vessel vasculitis). Cutaneous vasculitis presents as a constellation of clinical and histopathologic findings since the size of vessel involved correlates with the clinical findings.1,2,3,4 A diagnosis of cutaneous vasculitis is best determined by the histopathologic findings of the vessel size involved and the principal inflammatory response. Histopathologic findings are also crucial for distinguishing authentic cutaneous vasculitis from cutaneous pseudovasculitis7 (a heterogeneous collection of disorders that clinically produce hemorrhages [i.e., petechiae, purpura, and ecchymoses] or vessel occlusion [which results in livedo, cyanosis, ulcers, digital necrosis, and/or gangrene] and simulate cutaneous vasculitis but without the histopathologic evidence of vasculitis).7 This chapter will describe the morphological approach to cutaneous vasculitis, including: 1. The diagnostic criteria for small vessel vasculitis and for subcutaneous muscular vessel vasculitis (e.g., arteritis and phlebitis); 2. The details of the inflammatory stages of muscular vessel vasculitis (based on morphological changes during the inflammatory process, which strongly affect the treatment for vasculitis); 3. The histopathologic classification of cutaneous vasculitis (based on the size of the affected vessel [e.g., small vessels or muscular vessels] and the predominant inflammatory cell types [e.g., neutrophils, eosinophils, lymphocytes, histiocytes]) and their related disorders. Distinguishing between subcutaneous muscular arteritis and phlebitis will be also discussed since the morphological features of the subcutaneous veins in the lower legs, such as a